Cold-rolled steel sheet having excellent spot weldability, and manufacturing method therefor

ABSTRACT

A cold-rolled steel sheet having excellent spot weldability suitable for use in vehicles, electric appliances, etc. is provided. The cold-rolled steel sheet has a steel composition containing, in mass%: C: 0.05% to 0.13%; Si: 0.05% to 2.0%; Mn: 1.5% to 4.0%; P: 0.05% or less; S: 0.005% or less; Al: 0.01% to 0.10%; Cr: 0.05% to 1.0%; Nb: 0.010% to 0.070%; Ti: 0.005% to 0.040%; and N: 0.0005% to 0.0065%, with a mass ratio Ti/N of Ti and N being 2.5 or more and 7.5 or less, and a balance being Fe and incidental impurities, wherein 70 mass % or more of Ti in steel exists as a precipitate, and 15 mass % or more of Nb in the steel exists as solute Nb, and a tensile strength is 980 MPa or more.

TECHNICAL FIELD

The disclosure relates to a cold-rolled steel sheet with a sheet thickness of 0.4 mm or more and 3.0 mm or less suitable for use in vehicles, electric appliances, etc., and particularly relates to a cold-rolled steel sheet having excellent spot weldability with a tensile strength of 980 MPa or more and a manufacturing method therefor.

BACKGROUND

In recent years, improved fuel efficiency of vehicles has become increasingly important for global environment protection, which has encouraged reductions in weight of automotive bodies. The most effective means for this is to strengthen the steel sheets used and reduce their sheet thickness. It is also important to improve the safety of vehicle occupants. Effective means for this is equally to strengthen the steel sheets used. For such steel sheet strengthening, conventionally the conditions of hot rolling and subsequent continuous annealing have been strictly managed while adding various alloying elements such as C and Mn in steel sheets.

When using cold-rolled steel sheets as an automotive member, typically the steel sheets that have been formed are joined by welding and made into a desired finished shape. To ensure excellent safety as an automotive body structure, not only the base material of the cold-rolled steel sheets but also the area including the weld metal and the heat-affected zone is required to have excellent mechanical property. A conventional measure to ensure excellent weld property as cold-rolled steel sheets for vehicles typically limits the addition amounts of alloying elements for enhancing quench hardenability such as C and Mn and the addition amounts of impurity elements for facilitating the microsegregation of welds such as P and S.

However, it is extremely difficult to achieve both tensile strength as high as 980 MPa or more and high spot weldability, as there is a trade-off between increasing strength and increasing spot weldability by the addition of alloying components such as C and Mn.

For example, in resistance spot welding used as a typical method of joining steel sheets for vehicles, the steel sheets are heated to the melting point and then quenched. As a result, the weld metal becomes a solidified martensite single-phase structure in coarse columnar form. The heat-affected zone heated to a temperature range of Ac₃ point or more (hereafter also referred to as “heat-affected zone of Ac₃ point or more”) also becomes a relatively coarse martensite structure. The weld metal and the heat-affected zone of Ac₃ point or more are therefore higher in hardness than the base material, and susceptible to embrittlement. Besides, the heat-affected zone heated only to a temperature range less than Ac₃ point (hereafter also referred to as “heat-affected zone less than Ac₃ point”) is likely to decrease in strength due to tempering effect, and tends to have a higher softening degree with respect to the base material when the base material has higher strength. The weld typically has a discontinuous shape unlike the base material, so that stress tends to concentrate and residual stress due to welding heat hysteresis is unavoidable. Especially in a high strength steel sheet, the discontinuity of strength in the area from the weld metal through the heat-affected zone to the base material is significant, and the fracture strength of the spot weld is likely to be lower than that of the base material.

CITATION LIST Patent Literatures

PTL 1: JP 2012-167338 A

PTL 2: JP 4530606 B2

PTL 3: JP 4883216 B2

PTL 4: JP 5142068 B2

PTL 5: JP 5323552 B2

SUMMARY Technical Problem

High strength steel sheets proposed in JP 2012-167338 A (PTL 1), JP 4530606 B2 (PTL 2), JP 4883216 B2 (PTL 3), JP 5142068 B2 (PTL 4), JP 5323552 B2 (PTL 5), and the like fail to achieve both high strength of 980 MPa or more in tensile strength and sufficiently improved spot weldability while ensuring sufficient economic efficiency and productivity.

It could therefore be helpful to provide a cold-rolled steel sheet having excellent spot weldability with a tensile strength of 980 MPa or more and an advantageous manufacturing method therefor, without increasing manufacturing cost or decreasing productivity.

In the disclosure, “excellent spot weldability” means that the cross tensile strength is 10 kN/spot or more and the failure mode is plug failure in a cross tensile test according to JIS Z 3137 (1999), and the difference ΔHV between the maximum and minimum values of Vickers hardness in the area from the weld metal portion to the base material portion is less than 120 in a spot weld section test according to JIS Z 3139 (2009).

Solution to Problem

As a result of conducting extensive study on the chemical components of a steel sheet, a manufacturing method, and various factors determining microstructure, we discovered the following:

(1) To achieve a tensile strength of 980 MPa or more, it is important to strictly adjust the chemical composition of the steel sheet and appropriately control the mass % ratio of Ti and N (Ti/N).

By appropriately controlling Ti/N, strengthening by crystal grain refinement and strengthening by precipitation are realized through the generation of TiN. Moreover, the generation of Nb nitride is suppressed to secure solute Nb in the annealing process, which produces an effect of delaying the progress of recrystallization during heating and contributes to higher strength of the steel sheet.

(2) To achieve excellent spot weldability, it is important to suppress the embrittlement of the weld metal and heat-affected zone of Ac₃ point or more and also suppress the softening of the heat-affected zone less than Ac₃ point.

To suppress the embrittlement of the weld metal and heat-affected zone of Ac₃ point or more, it is necessary to minimize solute N, refine crystal grains, and suppress excessive hardening in the weld metal and heat-affected zone.

Moreover, when an appropriate amount of solute Nb exists in the steel, NbC is formed in the low-temperature range in the cooling process during welding, thus suppressing softening in the heat-affected zone less than Ac₃ point.

(3) To effectively produce the aforementioned effects, the existence states of Ti and Nb in the cold-rolled steel sheet after annealing need to be appropriately controlled.

To attain the desired existence states of Ti and Nb, it is important to strictly adjust the chemical composition of the steel sheet and Ti/N and appropriately control the manufacturing conditions, in particular the hot rolling conditions and the annealing conditions.

The disclosure is based on the aforementioned discoveries and further studies.

We provide the following:

1. A cold-rolled steel sheet having excellent spot weldability, the cold-rolled steel sheet having a steel composition containing (consisting of), in mass %: C: 0.05% to 0.13%; Si: 0.05% to 2.0%; Mn: 1.5% to 4.0%; P: 0.05% or less; S: 0.005% or less; Al: 0.01% to 0.10%; Cr: 0.05% to 1.0%; Nb: 0.010% to 0.070%; Ti: 0.005% to 0.040%; and N: 0.0005% to 0.0065%, with a mass ratio Ti/N of Ti and N being 2.5 or more and 7.5 or less, and a balance being Fe and incidental impurities, wherein 70 mass % or more of Ti in steel exists as a precipitate, and 15 mass % or more of Nb in the steel exists as solute Nb, and a tensile strength is 980 MPa or more.

2. The cold-rolled steel sheet having excellent spot weldability according to the foregoing 1, wherein the steel composition further contains one or more selected from, in mass %: Mo: 0.01% to 1.0%; Cu: 1.0% or less;

Ni: 1.0% or less; and V: 0.1% or less.

3. A manufacturing method for a cold-rolled steel sheet having excellent spot weldability, the manufacturing method including: heating a steel material having the steel composition according to the foregoing 1 or 2 to a temperature range of (Ts−50)° C. or more and (Ts+200)° C. or less where Ts is a temperature defined by the following Formula (1), hot rolling the steel material with a finisher delivery temperature of 850° C. or more to obtain a hot-rolled steel sheet, and then coiling the hot-rolled steel sheet at a temperature of 650° C. or less; cold rolling the hot-rolled steel sheet into a cold-rolled steel sheet; and continuously annealing the cold-rolled steel sheet by: heating the cold-rolled steel sheet to a temperature range of 700° C. or more and 900° C. or less; and, in a subsequent cooling process, cooling the cold-rolled steel sheet to a temperature range of 200° C. or more and 450° C. or less with an average cooling rate of 12° C./s or more and 100° C./s or less, and holding the cold-rolled steel sheet in the temperature range of 200° C. or more and 450° C. or less for a time of 30 s or more and 600 s or less,

Ts (° C.)=6770/[2.26−log ₁₀{[% Nb]×([% C]+0.86[% N])}]−273   (1)

where [% Nb], [% C], and [% N] respectively denote Nb, C, and N contents in steel in mass %.

Advantageous Effect

It is thus possible to obtain a cold-rolled steel sheet having excellent spot weldability with a tensile strength of 980 MPa or more, without increasing manufacturing cost or decreasing productivity.

The use of the cold-rolled steel sheet according to the disclosure improves manufacturing efficiency when producing steel structures such as vehicles and safety for vehicle occupants, and also improves fuel efficiency and thus significantly contributes to lower environmental burden.

DETAILED DESCRIPTION

Detailed description is given below.

The reasons for limiting the chemical composition of the steel material to the aforementioned range are described first. While the unit of the content of each element in the chemical composition of the steel material is “mass %”, the content is simply expressed in “%” unless otherwise stated.

C: 0.05% to 0.13%

C is the most important element in strengthening the steel, and has high solid solution strengthening ability. To achieve such effect, the C content needs to be 0.05% or more. If the C content is more than 0.13%, martensite phase in the base material increases and significantly hardens the material, causing degradation in hole expansion formability. The C content is therefore limited to the range of 0.05% to 0.13%. The C content is preferably in the range of 0.06% to 0.12%.

Si: 0.05% to 2.0%

Si is an element necessary in steelmaking, acting as a deoxidizing material. Si also has an effect of dissolving in the steel to strengthen the steel sheet by solid solution strengthening. To achieve such effects, the Si content needs to be 0.05% or more. If the Si content is more than 2.0%, the toughness of the weld metal and heat-affected zone degrades significantly, causing lower fracture strength of the weld. The Si content is therefore limited to the range of 0.05% to 2.0%. The Si content is preferably in the range of 0.10% to 1.60%.

Mn: 1.5% to 4.0%

Mn has an effect of increasing the quench hardenability of the steel at relatively low cost. To ensure a base material strength of 980 MPa or more in tensile strength, the Mn content needs to be 1.5% or more. If the Mn content is more than 4.0%, the fracture strength of the weld decreases, and the microsegregation of the base material increases, promoting a delayed fracture originating from the base material segregation area. The Mn content is therefore limited to the range of 1.5% to 4.0%. The Mn content is preferably in the range of 1.7% to 3.8%.

P: 0.05% or less

P is an element having high solid solution strengthening ability, but promotes microsegregation as with Mn. Accordingly, if the P content is more than 0.05%, not only the base material embrittles but also the grain boundary segregation area tends to become a delayed fracture origin. Hence, the P content is desirably minimized with the upper limit being 0.05%. Excessively reducing P, however, involves high refining cost and is economically disadvantageous. Therefore, the lower limit of the P content is desirably about 0.005%.

S: 0.005% or less

S segregates in the grain boundary and decreases ductility in hot rolling. Hence, the S content is desirably minimized with the upper limit being 0.005%.

Al: 0.01% to 0.10%

Al acts as a deoxidizer, and is the most generally used element in the molten steel deoxidizing process for steel sheets. Al also has an effect of fixing solute N in the steel to form AlN, thus suppressing embrittlement caused by solute N. To achieve such effects, the Al content needs to be 0.01% or more. If the Al content is more than 0.10%, surface cracking during slab manufacture is promoted. The Al content is therefore limited to the range of 0.01% to 0.10%. The Al content is preferably in the range of 0.02% to 0.07%.

Cr: 0.05% to 1.0%

Cr has an effect of increasing the quench hardenability of the steel at relatively low cost, and is an element that delays the bainite transformation of intermediate hardness phase in the annealing process and generates martensite of high hardness phase to contribute to improved strength of the steel. To achieve such effects, the Cr content needs to be 0.05% or more. If the Cr content is more than 1.0%, not only an excessive strength increase promotes embrittlement, but also an economic disadvantage is entailed. The Cr content is therefore limited to the range of 0.05% to 1.0%. The Cr content is preferably in the range of 0.07% to 0.8%.

Nb: 0.010% to 0.070%

Nb is an important element that, in annealing heating after cold rolling, exists as solute Nb to produce a solute drag effect and delay the recrystallization of the deformed microstructure generated in cold rolling, thus strengthening the steel sheet after annealing. Moreover, NbC generated in the hot rolling process and annealing process refines the microstructure in the base material and heat-affected zone, and improves toughness. To achieve such effects, the Nb content needs to be 0.010% or more. If the Nb content is more than 0.070%, coarse carbonitride precipitates, which promotes surface cracking during slag manufacture and may also become a fracture origin. The Nb content is therefore limited to the range of 0.010% to 0.070%. The Nb content is preferably in the range of 0.015% to 0.060%.

Ti: 0.005% to 0.040%

Ti is an important alloying element in the disclosure. By fixing solute N to form TiN, Ti has an effect of suppressing the coarsening of crystal grains in the weld metal and heat-affected zone and an effect of suppressing embrittlement by reducing solute N. Moreover, by forming TiN, Ti suppresses the generation of Nb nitride to secure a predetermined amount of solute Nb in the hot rolling and annealing steps, thus effectively contributing to higher strength of the steel sheet after annealing. To achieve such effects, the Ti content needs to be 0.005% or more. If the Ti content is more than 0.040%, very hard and brittle TiC precipitates, which promotes embrittlement. The Ti content is therefore limited to the range of 0.005% to 0.040%. The Ti content is preferably in the range of 0.010% to 0.035%.

N: 0.0005% to 0.0065%

N is contained in the steel as incidental impurity. However, when an appropriate amount of Ti is added, N forms TiN, and thus has an effect of suppressing the coarsening of crystal grains in the weld metal and heat-affected zone during welding. To achieve such effect, the N content needs to be 0.0005% or more. If the N content is more than 0.0065%, an increase of solute N causes a significant decrease in anti-aging property. The N content is therefore limited to the range of 0.0005% to 0.0065%. The N content is preferably in the range of 0.0010% to 0.0060%.

In the disclosure, it is important to appropriately control the mass% ratio of Ti and N, i.e. Ti/N, in addition to limiting the chemical composition as described above.

Ti/N: 2.5 or more and 7.5 or less

By controlling Ti/N in the aforementioned range, strengthening by crystal grain refinement and strengthening by precipitation are achieved through the generation of TiN. Moreover, an appropriate amount of solute Nb can be secured in the annealing process by suppressing the generation of Nb nitride, and the resulting effect of delaying the progress of recrystallization during heating contributes to higher strength of the steel sheet. The controlled ratio also contributes to reduced solute N and refined crystal grains in the weld metal and heat-affected zone, thus preventing the embrittlement of the weld metal and heat-affected zone.

If Ti/N is less than 2.5, solute N in the steel sheet increases, which promotes embrittlement. If Ti/N is more than 7.5, very hard and brittle TiC is generated in the steel sheet, causing lower ductility and significant embrittlement. Ti/N is therefore limited to the range of 2.5 to 7.5. Ti/N is preferably in the range of 3.0 to 7.0.

While the basic components have been described above, one or more selected from Mo, Cu, Ni, and V may also be contained according to need.

Mo: 0.01% to 1.0%

Mo is an element that contributes to improved strength of the steel. To achieve such effect, the Mo content needs to be 0.01% or more. If the Mo content is more than 1.0%, not only an excessive strength increase promotes embrittlement, but also an economic disadvantage is entailed. Accordingly, in the case of adding Mo, the Mo content is in the range of 0.01% to 1.0%. The Mo content is preferably in the range of 0.03% to 0.8%.

Cu: 1.0% or less

Cu is an element that contributes to improved strength of the steel. If the Cu content is more than 1.0%, however, hot shortness occurs and the surface characteristics of the steel sheet degrade. Accordingly, in the case of adding Cu, the Cu content is 1.0% or less.

Ni: 1.0% or less

Ni is an element that contributes to improved strength of the steel. If the Ni content is more than 1.0%, however, the effect saturates, which is economically disadvantageous. Accordingly, in the case of adding Ni, the Ni content is 1.0% or less.

V: 0.1% or less

V is an element that contributes to improved strength of the steel. If the V content is more than 0.1%, however, the ductility of the base material degrades. Accordingly, in the case of adding V, the V content is 0.1% or less.

In the chemical composition of the steel sheet according to the disclosure, the balance other than the aforementioned components is Fe and incidental impurities.

The chemical composition of the steel sheet according to the disclosure has been described above. In the disclosure, it is very important to appropriately control the existence forms of Ti and Nb in the steel.

Proportion of Ti existing as precipitate in steel: 70 mass % or more

In the annealing process, Ti precipitate refines the structure, thus improving the hole expansion formability of the eventually obtained cold-rolled steel sheet. In addition, when Ti exists as a precipitate in the cold-rolled steel sheet after annealing, the coarsening of crystal grains in the heat-affected zone due to welding heat hysteresis during welding is suppressed, so that the fracture strength of the weld is improved. To achieve such effects, 70 mass % or more of Ti in the steel need to exist as a precipitate. The proportion of Ti existing as a precipitate in the steel is preferably 75 mass % or more. The upper limit of the proportion of Ti existing as a precipitate in the steel is not particularly limited. If the proportion is 100 mass %, however, toughness degrades significantly due to remaining solute N. Accordingly, the proportion of Ti existing as a precipitate in the steel is preferably less than 100 mass %, and more preferably less than 98 mass %.

The form of the precipitate is mainly a single precipitate of TiN or a composite precipitate of TiN and another precipitate. Even when Ti oxide or Ti carbide is mixed, its effect is negligible as long as Ti oxide or Ti carbide is less than 10% of the total number of Ti-based precipitates. The existence form of Ti in the steel other than a precipitate is solute Ti.

Proportion of Nb existing as solute Nb in steel: 15 mass % or more

Nb existing as a solute has an effect of suppressing recrystallization during heating in the annealing process to effectively contribute to higher strength of the steel and also has an effect of suppressing the softening of the heat-affected zone less than Ac₃ point.

To achieve such effects, 15 mass % or more of Nb in the steel need to exist as solute Nb. The proportion of Nb existing as solute Nb in the steel is preferably 20 mass % or more.

The upper limit of the proportion of Nb existing as solute Nb in the steel is not particularly limited. If the amount of solute Nb in the steel is excessively high, however, the aforementioned effects saturate, and the manufacturing cost increases. Accordingly, the proportion of Nb existing as solute Nb in the steel is preferably 70 mass % or less.

The existence form of Nb in the steel other than solute Nb is Nb precipitate. Examples of the Nb precipitate include Nb carbide and Nb carbonitride such as NbC.

The following describes a manufacturing method according to the disclosure. Note that the temperature of the steel sheet in the manufacturing conditions is the surface temperature of the steel sheet.

Molten steel having the aforementioned chemical composition is obtained by steelmaking using a known method such as a converter or an electric heating furnace, and made into a steel material such as a slab having predetermined dimensions using a known method such as continuous casting or ingot casting and blooming. The molten steel may also be subjected to treatment such as refining with a ladle or vacuum degassing.

The obtained steel material is immediately or temporarily cooled, heated to a temperature range of (Ts−50)° C. or more and (Ts+200)° C. or less, and hot rolled with a finisher delivery temperature of 850° C. or more. The steel material is then coiled at 650° C. or less, to form a hot-rolled steel sheet.

Here, Ts is defined by the following Formula (1):

Ts (° C.)=6770/[2.26−log ₁₀{[% Nb]×([% C]+0.86[% N])}]−273   (1)

where [% Nb], [% C], and [% N] respectively denote the Nb, C, and N contents (mass %) in the steel.

Heating temperature: (Ts−50)° C. or more and (Ts+200)° C. or less

Carbonitride containing coarse Nb which has crystallized during the steelmaking of the steel material does not contribute to higher strength of the steel sheet. It is therefore important to temporarily dissolve such coarse Nb-based crystallized product in the steel in the heating stage before hot rolling, and precipitate it again as fine Nb carbide or carbonitride in the subsequent processes such as rolling, cooling, and annealing.

If the heating temperature is less than (Ts−50)° C., heating is insufficient and the Nb-based crystallized product does not sufficiently dissolve in the steel, leading to insufficient strength after annealing. If the heating temperature is more than (Ts+200)° C., the aforementioned effects saturate. Besides, the Ti crystallized product dissolves completely, making it difficult to cause an appropriate amount of Ti to exist as a precipitate after annealing. Further, the fuel cost for heating increases and also the yield rate drops due to increased scale-off, which is economically disadvantageous.

The heating temperature is therefore (Ts−50)° C. or more and (Ts+200)° C. or less. The heating temperature is preferably (Ts−20)° C. or more and (Ts+170)° C. or less.

Finisher delivery temperature: 850° C. or more

If the finisher delivery temperature is less than 850° C., not only rolling efficiency drops, but also the rolling load increases, causing a greater load on the mill. The finisher delivery temperature is therefore 850° C. or more.

Coiling temperature: 650° C. or less

If the coiling temperature for the hot-rolled steel sheet is more than 650° C., NbC which precipitates during coiling coarsens excessively, which facilitates embrittlement and is likely to provide a fracture origin. The coiling temperature for the hot-rolled steel sheet therefore needs to be 650° C. or less. The coiling temperature for the hot-rolled steel sheet is preferably 620° C. or less. The lower limit of the coiling temperature for the hot-rolled steel sheet need not be particularly limited. Given that an excessive temperature decrease causes lower manufacturing efficiency, however, the lower limit is preferably about 400° C.

The obtained hot-rolled steel sheet is then cold rolled into a cold-rolled steel sheet. The cold rolling conditions need not be particularly limited. To ensure desired strength after annealing, however, the total rolling reduction is preferably 30% or more. Moreover, to avoid an excessive load on the mill, the total rolling reduction is preferably 80% or less.

The cold-rolled steel sheet obtained in this way is then continuously annealed under the following conditions.

Heating temperature in continuous annealing: 700° C. or more and 900° C. or less

If the heating temperature in continuous annealing is less than 700° C., the reverse transformation of austenite is insufficient, and the amount of hard martensite or bainite generated in the subsequent cooling is insufficient, making it impossible to obtain desired strength. If the heating temperature in continuous annealing is more than 900° C., austenite grains coarsen considerably, causing degradation in hole expansion formability of the base material and toughness of the heat-affected zone. The heating temperature in continuous annealing is therefore 700° C. or more and 900° C. or less. The heating temperature in continuous annealing is preferably 720° C. or more and 880° C. or less.

The holding time after heating need not be particularly limited. To ensure a uniform temperature distribution and a stable microstructure, however, the holding time is preferably 15 s or more. Meanwhile, a long holding time causes not only lower manufacturing efficiency but also coarser austenite grains, and so the holding time is preferably 600 s or less.

Average cooling rate: 12° C./s or more and 100° C./s or less

If the average cooling rate in the cooling process after heating in continuous annealing is less than 12° C./s, soft ferrite phase is generated excessively during cooling, making it difficult to ensure desired strength. Besides, Nb reprecipitates excessively in the middle of cooling, making it difficult to secure a desired amount of solute Nb. Further, coarse ferrite phase or pearlite phase is generated in the middle of cooling, leading to a decrease in strength. If the average cooling rate after annealing is more than 100° C./s, it is difficult to secure the shape of the steel sheet. The average cooling rate after annealing treatment is therefore 12° C./s or more and 100° C./s or less. The average cooling rate is preferably 14° C./s or more and 70° C./s or less.

Cooling stop temperature: 200° C. or more and 450° C. or less

If the cooling stop temperature is less than 200° C., the conveyance speed for the steel sheet is to be lowered extremely, which is not preferable in terms of manufacturing efficiency. If the cooling stop temperature is more than 450° C., relatively soft bainite phase is generated excessively after the cooling stop, making it difficult to ensure desired strength. Besides, Nb reprecipitates excessively after the cooling stop, making it difficult to secure a desired amount of solute Nb. Further, a soft structure such as ferrite is generated excessively, leading to insufficient strength. The cooling stop temperature is therefore 200° C. or more and 450° C. or less. The cooling stop temperature is preferably 230° C. or more and 420° C. or less.

Holding time in cooling stop temperature range: 30 s or more and 600 s or less

If the holding time in the cooling stop temperature range is less than 30 s, the uniformity of the temperature and material in the steel sheet decreases. If the holding time in the cooling stop temperature range is more than 600 s, manufacturing efficiency decreases. The holding time in the cooling stop temperature range is therefore 30 s or more and 600 s or less.

EXAMPLES

Steel having the chemical composition shown in Table 1 was obtained by steelmaking using a converter, refined with a ladle, and continuously cast into a steel slab. The steel slab was then hot rolled under the conditions shown in Table 2, into a hot-rolled steel sheet. The hot-rolled steel sheet was cold rolled and continuously annealed under the conditions shown in Table 2, thus obtaining a cold-rolled steel sheet as a product sheet.

TABLE 1 Steel Chemical composition (mass %) No. C Si Mn P S Al Cr Nb Ti N 1 0.074 0.52 2.86 0.013 0.0011 0.029 0.16 0.029 0.018 0.0044 2 0.114 1.46 1.84 0.005 0.0010 0.033 0.65 0.037 0.015 0.0036 3 0.083 0.22 3.10 0.019 0.0024 0.035 0.21 0.051 0.011 0.0029 4 0.098 0.13 1.96 0.025 0.0031 0.051 0.39 0.038 0.028 0.0052 5 0.106 0.83 3.28 0.010 0.0019 0.065 0.08 0.041 0.010 0.0016 6 0.119 0.26 2.76 0.008 0.0012 0.036 0.42 0.029 0.032 0.0060 7 0.067 1.12 3.74 0.036 0.0029 0.066 0.14 0.057 0.023 0.0046 8 0.124 0.28 2.41 0.012 0.0018 0.023 0.23 0.019 0.021 0.0056 9 0.039 0.58 2.71 0.024 0.0028 0.031 0.13 0.028 0.025 0.0046 10 0.165 0.49 3.32 0.012 0.0024 0.030 0.20 0.045 0.015 0.0029 11 0.076 0.44 1.01 0.008 0.0014 0.022 0.24 0.030 0.012 0.0036 12 0.095 0.32 4.38 0.021 0.0031 0.041 0.32 0.029 0.028 0.0046 13 0.072 0.21 2.41 0.012 0.0028 0.048 0.01 0.021 0.015 0.0047 14 0.075 0.32 2.59 0.016 0.0015 0.025 0.17 0.008 0.013 0.0038 15 0.118 0.99 3.51 0.031 0.0033 0.050 0.20 0.042 0.004 0.0012 16 0.068 0.25 3.44 0.016 0.0029 0.032 0.26 0.021 0.041 0.0060 17 0.120 0.16 3.20 0.021 0.0025 0.030 0.09 0.049 0.033 0.0076 18 0.093 0.38 3.02 0.015 0.0012 0.046 0.35 0.024 0.014 0.0058 19 0.111 0.30 3.39 0.031 0.0030 0.026 0.15 0.041 0.032 0.0022 Steel Chemical composition (mass %) Ts Ts − 50 Ts + 200 No. Mo Cu Ni V Ti/N (° C.) (° C.) (° C.) Remarks 1 — — — — 4.1 1107 1057 1307 Conforming steel 2 — — — — 4.2 1191 1141 1391 Conforming steel 3 — — — — 3.8 1192 1142 1392 Conforming steel 4 0.36 — — — 5.4 1177 1127 1377 Conforming steel 5 — 0.12 0.19 — 6.3 1194 1144 1394 Conforming steel 6 — — — 0.05 5.3 1166 1116 1366 Conforming steel 7 0.06 — — — 5.0 1182 1132 1382 Conforming steel 8 — — — — 3.8 1117 1067 1317 Conforming steel 9 — — — — 5.4 1034 984 1234 Comparative steel 10 — — — 0.05 5.2 1272 1222 1472 Comparative steel 11 — 0.18 — — 3.3 1113 1063 1313 Comparative steel 12 — — — — 6.1 1137 1087 1337 Comparative steel 13 — — — — 3.2 1066 1016 1266 Comparative steel 14 — — — 0.04 3.4 966 916 1166 Comparative steel 15 0.28 — — — 3.3 1211 1161 1411 Comparative steel 16 — — — — 6.8 1061 1011 1261 Comparative steel 17 0.10 — — — 4.3 1242 1192 1442 Comparative steel 18 — — 0.20 — 2.4 1112 1062 1312 Comparative steel 19 — — — — 14.5  1200 1150 1400 Comparative steel Underlines indicate outside the appropriate range.

TABLE 2 Cold rolling Annealing conditions Ma- Hot rolling conditions conditions Cooling terial Heating Finisher Coiling Sheet Total Sheet Heating Heating Cool- stop Hold- thick- temper- delivery temper- thick- rolling thick- temper- holding ing temper- ing Steel ness ature temperature ature ness reduction ness ature time rate ature time No. No. (mm) (° C.) (° C.) (° C.) (mm) (%) (mm) (° C.) (s) (° C./s) (° C.) (s) Remarks 1-1  1 200 1200 900 590 2.8 50 1.4 790 100 15 320 200 Example 1-2 200 1200 860 590 2.8 50 1.4 790 100 15 280 500 Example 1-3 200 1030 860 590 2.8 50 1.4 790 100 15 300 200 Comparative Example 1-4 200 1330 930 590 2.8 50 1.4 790 100 15 300 200 Comparative Example 1-5 200 1200 900 700 2.8 50 1.4 790 100 15 300 200 Comparative Example 1-6 200 1200 900 590 2.8 50 1.4 920 100 15 300 200 Comparative Example 1-7 200 1200 900 590 2.8 50 1.4 680 100 15 300 200 Comparative Example 2  2 210 1200 890 600 2.8 50 1.4 820 60 15 270 120 Example 3-1  3 200 1230 900 600 2.8 50 1.4 780 80 25 310 150 Example 3-2 200 1230 900 600 2.8 50 1.4 780 80  3 310 150 Comparative Example 3-3 200 1230 900 600 2.8 50 1.4 780 80 20 480 150 Comparative Example 4  4 200 1200 870 520 2.8 50 1.4 820 70 70 230 150 Example 5  5 200 1250 880 600 2.8 50 1.4 760 150 13 310 400 Example 6  6 200 1280 900 620 2.8 50 1.4 880 90 20 330 200 Example 7  7 200 1150 920 600 2.8 50 1.4 750 90 20 290 200 Example 8  8 200 1100 860 450 2.8 50 1.4 770 90 25 300 120 Example 9  9 200 1200 880 580 2.8 50 1.4 780 90 15 300 200 Comparative Example 10 10 230 1280 920 550 2.8 50 1.4 810 60 40 250 120 Comparative Example 11 11 200 1220 880 560 2.8 50 1.4 830 90 30 300 180 Comparative Example 12 12 200 1250 900 600 2.8 50 1.4 780 60 20 300 120 Comparative Example 13 13 220 1200 860 550 2.8 50 1.4 840 100 20 310 230 Comparative Example 14 14 200 1150 850 500 2.8 50 1.4 810 100 25 280 200 Comparative Example 15 15 200 1250 930 600 2.8 50 1.4 810 90 20 300 180 Comparative Example 16 16 200 1250 900 560 2.8 50 1.4 800 120 15 280 300 Comparative Example 17 17 200 1250 900 600 2.8 50 1.4 790 100 20 300 250 Comparative Example 18 18 200 1200 900 550 2.8 50 1.4 780 60 25 320 100 Comparative Example 19 19 200 1250 900 600 2.8 50 1.4 800 90 20 290 150 Comparative Example Underlines indicate outside the appropriate range.

Each cold-rolled steel sheet obtained as a result was subjected to (1) analysis of extracted residue of precipitate, (2) tensile test, and (3) spot weld test as follows.

(1) Analysis of Extracted Residue of Precipitate

An electroextraction test piece was collected from each cold-rolled steel sheet obtained as mentioned above, and subjected to electrolytic treatment using a AA electrolytic solution (ethanol solution of acetylacetone tetramethylammonium chloride), to extract a residue by filtration.

The extracted residue was set to a constant volume of 100 ml using pure water, and the amount of Ti was measured by high-frequency inductively coupled plasma (ICP) emission spectrometry as the amount of Ti existing as a precipitate. Likewise, the amount of Nb in the extracted residue was measured, and the measured amount of Nb was subtracted from the total amount of Nb in the test piece to calculate the amount of solute Nb.

The calculated amount of Ti existing as a precipitate and amount of solute Nb were respectively divided by the total amount of Ti and total amount of Nb in the test piece, to find the proportion of Ti existing as a precipitate in the steel and the proportion of Nb existing as solute Nb in the steel. The evaluation results are shown in Table 3.

(2) Tensile Test

A JIS No. 5 tensile test piece was collected in the direction orthogonal to the rolling direction, and tensile strength (TS) and total elongation (El) were measured according to JIS Z 2241 (2011). The evaluation results are shown in Table 3. Each sample with TS 980 MPa and El 13% was determined as favorable.

(3) Spot Weld Test

Cross Tensile Test

Each cold-rolled steel sheet obtained as mentioned above was used to form a cross tensile test piece according to JIS Z 3137 (1999). Spot welding in the formation of the cross tensile test piece was performed under the welding conditions of a nugget diameter of 6.0 mm according to the Japan Welding Engineering Society Standard: WES7301.

The formed cross tensile test piece was then subjected to a cross tensile test according to JIS Z 3137 (1999). Each sample with a cross tensile strength of 10 kN/spot or more and a failure mode of plug failure was determined as excellent in spot weldability.

Section Test

A section test was conducted according to JIS Z 3139 (2009).

Two cold-rolled steel sheets of the same steel sample ID were spot welded under the same conditions as the aforementioned cross tensile test piece forming conditions. After polishing a weld section cut perpendicularly to the steel sheet surface, nital etching was applied to obtain a hardness measurement test piece. According to JIS Z 2244 (2009), a Vickers hardness test was conducted from the weld metal portion to the base material portion with a pitch of 0.5 mm from the center position of the nugget in two directions parallel to the steel sheet surface at the positions of 0.5 mm above and 0.5 mm below the center position in the sheet thickness direction, with a test force of 0.9807 N. The difference (4HV) between the maximum and minimum values of the measured Vickers hardness was then calculated. Each sample with ΔHV of less than 120 was determined as excellent in spot weldability.

The evaluation results are shown in Table 3.

TABLE 3 (1) Analysis result of extracted (3) Spot weld test result residue of precipitate (2) Tensile (Joint hardness distribution) Proportion of Proportion of test result Cross tensile Difference between maximum and precipitate Ti solute Nb TS El strength minimum values of Vickers hardness No. (mass %) (mass %) (MPa) (%) (kN/spot) Failure mode ΔHV Remarks 1-1 85.1 26.9 1039  16.4 12.0 Plug failure 77 Example 1-2 89.4 23.6 1082  15.2 11.6 Plug failure 80 Example 1-3 86.2 12.5 906 20.1 9.1 Plug failure 126 Comparative Example 1-4 51.4 48.6 1036  12.7 11.6 Plug failure 82 Comparative Example 1-5 95.2 10.4 948 18.1 9.8 Plug failure 125 Comparative Example 1-6 82.1 33.6 954 12.0 11.6 Plug failure 80 Comparative Example 1-7 69.4 10.8 789 20.3 8.7 Plug failure 139 Comparative Example 2 85.4 24.9 990 17.6 11.5 Plug failure 10.1 Example 3-1 90.1 24.3 1032  16.2 11.4 Plug failure 88 Example 3-2 93.2 10.7 852 18.8 9.6 Plug failure 135 Comparative Example 3-3 89.2 12.6 931 18.0 9.7 Plug failure 128 Comparative Example 4 80.6 60.8 991 16.0 11.7 Plug failure 94 Example 5 88.4 33.4 1098  13.6 10.5 Plug failure 72 Example 6 91.7 16.8 1028  16.7 11.4 Plug failure 110 Example 7 72.7 21.3 987 14.3 10.6 Plug failure 101 Example 8 87.4 24.1 992 16.2 11.1 Plug failure 90 Example 9 76.4 55.2 812 19.6 8.7 Plug failure 57 Comparative Example 10 78.4 12.7 1157  10.9 9.1 Interface failure 151 Comparative Example 11 77.5 19.3 882 17.4 9.2 Plug failure 135 Comparative Example 12 80.4 35.5 1162  11.8 9.1 Interface failure 125 Comparative Example 13 82.5 33.3 942 17.2 10.4 Plug failure 59 Comparative Example 14 82.1 69.4 862 17.1 8.9 Plug failure 121 Comparative Example 15 66.9  8.2 830 16.7 8.2 Plug failure 130 Comparative Example 16 89.2 36.1 1096  10.2 8.2 Interface failure 111 Comparative Example 17 92.3 11.9 973 18.4 9.5 Interface failure 127 Comparative Example 18 98.6 12.3 955 17.9 9.9 Plug failure 106 Comparative Example 19 52.4 38.6 1102  11.8 9.7 Plug failure 101 Comparative Example Underlines indicate outside the appropriate range.

As shown in Table 3, all Examples had a tensile strength of 980 MPa or more, and had excellent spot weldability as the cross tensile strength was 10 kN/spot or more, the failure mode was plug failure, and the difference 4HV between the maximum and minimum values of Vickers hardness was less than 120. All Examples also had a total elongation of 13% or more.

On the other hand, Comparative Examples had insufficient performance in at least one of the tensile strength and total elongation of the base material and the cross tensile strength, the failure mode, and the difference (ΔHV) between the maximum and minimum values of Vickers hardness in the spot weld test. 

1. A cold-rolled steel sheet having excellent spot weldability, the cold-rolled steel sheet having a steel composition containing, in mass %: C: 0.05% to 0.13%; Si: 0.05% to 2.0%; Mn: 1.5% to 4.0%; P: 0.05% or less; S: 0.005% or less; Al: 0.01% to 0.10%; Cr: 0.05% to 1.0%; Nb: 0.010% to 0.070%; Ti: 0.005% to 0.040%; and N: 0.0005% to 0.0065%, with a mass ratio Ti/N of Ti and N being 2.5 or more and 7.5 or less, and a balance being Fe and incidental impurities, p1 wherein 70 mass % or more of Ti in steel exists as a precipitate, and 15 mass % or more of Nb in the steel exists as solute Nb, and a tensile strength is 980 MPa or more.
 2. The cold-rolled steel sheet having excellent spot weldability according to claim 1, wherein the steel composition further contains one or more selected from, in mass %: Mo: 0.01% to 1.0%; Cu: 1.0% or less; Ni: 1.0% or less; and V: 0.1% or less.
 3. A manufacturing method for a cold-rolled steel sheet having excellent spot weldability, the manufacturing method comprising: heating a steel material having the steel composition according to claim 1 to a temperature range of (Ts−50)° C. or more and (Ts+200)° C. or less where Ts is a temperature defined by the following Formula (1), hot rolling the steel material with a finisher delivery temperature of 850° C. or more to obtain a hot-rolled steel sheet, and then coiling the hot-rolled steel sheet at a temperature of 650° C. or less; cold rolling the hot-rolled steel sheet into a cold-rolled steel sheet; and continuously annealing the cold-rolled steel sheet by: heating the cold-rolled steel sheet to a temperature range of 700° C. or more and 900° C. or less; and, in a subsequent cooling process, cooling the cold-rolled steel sheet to a temperature range of 200° C. or more and 450° C. or less with an average cooling rate of 12° C./s or more and 100° C./s or less, and holding the cold-rolled steel sheet in the temperature range of 200° C. or more and 450° C. or less for a time of 30 s or more and 600 s or less, Ts (° C.)=6770/[2.26−log ₁₀{[% Nb]×([% C]+0.86[% N])}]−273   (1) where [% Nb], [% C], and [% N] respectively denote Nb, C, and N contents in steel in mass %.
 4. A manufacturing method for a cold-rolled steel sheet having excellent spot weldability, the manufacturing method comprising: heating a steel material having the steel composition according to claim 2 to a temperature range of (Ts−50)° C. or more and (Ts+200)° C. or less where Ts is a temperature defined by the following Formula (1), hot rolling the steel material with a finisher delivery temperature of 850° C. or more to obtain a hot-rolled steel sheet, and then coiling the hot-rolled steel sheet at a temperature of 650° C. or less; cold rolling the hot-rolled steel sheet into a cold-rolled steel sheet; and continuously annealing the cold-rolled steel sheet by: heating the cold-rolled steel sheet to a temperature range of 700° C. or more and 900° C. or less; and, in a subsequent cooling process, cooling the cold-rolled steel sheet to a temperature range of 200° C. or more and 450° C. or less with an average cooling rate of 12° C./s or more and 100° C./s or less, and holding the cold-rolled steel sheet in the temperature range of 200° C. or more and 450° C. or less for a time of 30 s or more and 600 s or less, Ts (° C.)=6770/[2.26−log ₁₀{[% Nb]×([% C]+0.86[% N])}]−273   (1) where [% Nb], [% C], and [% N] respectively denote Nb, C, and N contents in steel in mass %. 